1. Field of the Invention
The present invention generally relates to oscillators, and more particularly, to an oscillator having at least one resonance circuit.
2. Description of the Related Art
Oscillators, particularly, voltage-controlled oscillators have been used in mobile communication equipment such as PHS (Personal Handyphone System), wireless LAN (Local Area Network), and transceivers.
The voltage-controlled oscillator includes a resonance circuit generating a resonance signal of a frequency controlled by a control voltage, and an oscillation circuit generating an oscillation signal of the resonance frequency.
FIG. 1 is a circuit diagram of a conventional oscillator. The oscillator includes an oscillation unit 11, which includes a resonance circuit 25, an oscillation circuit 30 and a buffer circuit 40. A transistor Q1 that forms the oscillation circuit 30 and a transistor Q2 that forms the buffer circuit 40 are cascaded between ground and a power supply terminal Tb.
The resonance circuit 25 includes a variable capacitance diode D1, capacitors C2 and C3 and a line S5 such as a strip line. A control voltage applied to a control terminal Ta is processed by a line S4 and a capacitor C1 so that high-frequency components of the control voltage can be removed, and is then applied to the variable capacitance diode D1. The electrostatic capacitance of the variable capacitance diode D1 is changed by the control voltage, and the changed electrostatic capacitance changes the resonance frequency of the resonance circuit 25.
The oscillation circuit forms a Clap oscillation circuit that is composed of a bipolar transistor Q1 and the capacitors C5 and C6 and is a kind of Colpitts oscillator. The base of the transistor Q1 is connected to the resonance circuit 25 via a coupling capacitor C4.
The buffer circuit 40 includes a bipolar transistor Q2. The emitter of the bipolar transistor Q2 is connected to the collector of the transistor Q1 of the oscillation circuit 30, and the base of the transistor Q2 is connected to the emitter of the transistor Q1 via a coupling capacitor C8.
A line S2 functions as a choke inductor that removes AC components when power is applied to the collector of the transistor Q2 from the power supply terminal Tb. Resistors R1 through R4 defines a bias supplied to the transistors Q1 and Q2. Capacitors C7, C10 and C11 function to short-circuit high-frequency components to ground.
The voltage-controlled oscillator is required to have reduced phase noise and an enhanced C/N (Carrier to Noise) ratio. Increased output power may enhance the C/N ratio, but may increase power consumption.
Japanese Patent Application Publication No. 2003-188723 discloses a voltage-controlled oscillator in which a control voltage is applied to the cathodes of three variable capacitance diodes.